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Tail water elevations in HydroCAd

Tail water elevations in HydroCAd

(OP)
If you have different variable tail water elevations corresponding to different storm events, can you use the same link node in HydroCAD? It seems like you should, as the elevation tab lets you select different events, however when as soon as I define a second event, it overwrites the first. For instance, I select 2-year event, define the tail water, and hit apply. Go to define a 100-year tail water. The 100-year has now overwritten the previously entered two year. Obviously the work around would be to create a link for each event, but it just seems that you can't use the same link node. Using HydroCAD 10.00-16. Thanks.

RE: Tail water elevations in HydroCAd

If you use a link to define a manual tailwater elevation, the data is applied to all events. The event selection you refer to applies only to a manual hydrograph entry.

Your best solution is to model the tailwater point within HydroCAD using an appropriate reach or pond, so that you have the correct elevation vs. time for each event. Remember that the elevation is constantly varying throughout the storm, so entering a single (fixed) tailwater will rarely produce an accurate model.

Peter Smart
HydroCAD Software
www.hydrocad.net

RE: Tail water elevations in HydroCAd

(OP)
This is what I was referring to in the link tab. You can't use this to define elevations for different events without the using a hydrograph?

RE: Tail water elevations in HydroCAd

That option allows the tailwater to be applied to a specific event. If you specify the event, you can only define the elevation for that ONE event. A link cannot be used to specify a different elevation for each event. To do that you need to model that point in the system so that HydroCAD can automatically calculate the tailwater.

Peter Smart
HydroCAD Software
www.hydrocad.net

RE: Tail water elevations in HydroCAd

Peter-

It appears this would be a great "wish list" item: the ability to have a different tailwater curve for each storm event in a single link.

RE: Tail water elevations in HydroCAd

(OP)
Okay. Thanks Peter.

RE: Tail water elevations in HydroCAd

Yup, it's on the wish-list.

However, I would advise caution when using a manual tailwater. For correct results, you need a complete table of elevation vs. time, and not just a fixed elevation. Using a fixed elevation (such as the maximum for each event) will generally cause the site discharge to be too low before and after the time of peak tailwater. A fixed tailwater is only appropriate when the receiving water body is stable during the entire duration of your model, otherwise you need to include the receiving body in your model in order to calculate an accurate tailwater elevation vs. time.

Peter Smart
HydroCAD Software
www.hydrocad.net

RE: Tail water elevations in HydroCAd

While that's absolutely true, Peter, a lot of times we're stuck with a fixed tailwater elevation for a pond, that we're required to take into account. Most commonly, when a pond discharges to a floodplain and the HEC-RAS model for the floodplain is steady state. Steady state models are pretty much the norm as far as I've seen. It's common for me to have a list of relevant flood elevations from a steady state HEC-RAS model for different event storms, but no hydrograph, and no good way to generate one since the drainage area is beyond the scope of TR-20.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

RE: Tail water elevations in HydroCAd

beej67 - I completely understand. I just want everyone to realize the consequences of using a fixed TW if it's not actually fixed throughout the typical 24-hour storm duration.

Peter Smart
HydroCAD Software
www.hydrocad.net

RE: Tail water elevations in HydroCAd

That's why I would love to be able to put in a full time-stage curve that is linked to each storm event rather than have a fixed tailwater or having to have multiple files to cover the curves for each storm event.

RE: Tail water elevations in HydroCAd

A third work-around, Terry, would be to copy/paste the nodes in the model side by side and fix the routing arrows. They would all be in the same model, but you'd have to look at different nodes for different return periods. Some of the summary tables would look a little goofy, in that they'd be double counting areas, but the hydraulic results would be good. I often do this with pre and post development detention pond models, so I can easily compare data between the two models.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

RE: Tail water elevations in HydroCAd

beej67,

Yes, I do the same as a work-around. It is just more cumbersome and can be prone to human error (forgetting to switch the routing), and, as you stated, can look peculiar in reports for reviewers.

RE: Tail water elevations in HydroCAd

You can expect to have multi-event link capability in the next HydroCAD 10.0 build 17, coming soon...

Peter Smart
HydroCAD Software
www.hydrocad.net

RE: Tail water elevations in HydroCAd

Neat.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

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